1. Field of the Invention
The present invention relates to a signal processing circuit which is arranged to perform by a single A/D-converter analog-to-digital (A/D) conversion of two kinds of analog signals that differ in reference voltages thereof.
2. Description of Related Art
Conventionally, in an engine control unit (ECU) for use in, e.g. an automobile, as illustrated in FIG. 30, there are provided knock sensors 1 and 2 and the like for detecting a knock in addition to various sensors for detecting operational states or parameters of the engine such as cooling water temperature of engine, intake air temperature, throttle opening, etc. In order to perform calculation processings of the output signals from these various sensors by CPU 3, it is necessary to A/D-convert the sensor output signals (analog value) to digital values through the operation of A/D-converters 4 and 5. Each of those various sensors for outputting a voltage signal that corresponds to the operational states of the engine such as cooling water temperature thereof uses a reference voltage of from 0V to 5V in conformity with the operating voltage of the CPU 3, the output voltage therefrom varying within a range of from 0V to 5V. For this reason, the A/D-converter 4 for performing A/D-conversion with respect to such output voltage also uses the same reference voltage (0V-5V per 10 bits) as that of each sensor.
Meanwhile, since each of the knock sensors 1 and 2 is arranged to detect a knock by the use of the amplitude of its output voltage (knock signal), it uses as a reference value ("0" point value) an intermediate voltage of 2.5V of the operating voltage (0V-5V) of the CPU 3. Accordingly, the knock signal that has only necessary knock vibration frequency components taken out by a band pass filter 6 becomes a signal which changes around a voltage of 2.5V. The maximum value (the peak value) of this knock signal is held in a peak-hold circuit (P/H circuit) 7, whereby the occurrence of a knock is determined according to the magnitude of the peak value. Further, the input value of the knock signal that is supplied to the A/D-converter 5 is held by a sample hold circuit (S/H circuit) 8, whereby in units of, for example, 50 .mu.s the output signal from the S/H circuit 8 is A/D-converted and cumulative calculation is performed of the thus A/D-converted value through the operation of the CPU 3. According to the resulting value thus obtained, detection is performed of a malfunction such as wire breakage of the knock sensors 1 and 2.
As mentioned above, in order to detect the knock signal that changes around 2.5V, it is arranged that the upper limit of the output voltage of each of the P/H circuit 7 and S/H circuit 8 is set to be 4.5V and the reference voltage value thereof is set to be 2.5V. Therefore, the A/D-converter 5 for A/D-converting the output voltage of each of these P/H circuit 7 and S/H circuit 8 is constituted by one whose reference voltage ranges from 2.5V to 4.5V (8 bits). By the reference voltages of the both circuits being made to coincide with each other, the output voltages of the P/H circuit 7 and S/H circuit 8 can be A/D-converted with high accuracy without being influenced by the variations of the reference voltages.
In the above-mentioned conventional circuit construction, since the use of the two A/D-converters 4 and 5 is needed due to the difference in reference voltages, for example, between the knock signal and other sensor signals, the circuit construction disadvantageously becomes complex and becomes expensive. As a countermeasure against this, it is been considered to simplify the circuit construction by making these two A/D-converters 4 and 5 common by the use of a single general purpose A/D-converter (0V-5V per 10 bits).
In even this case, while the knock signal has peak held its value at which the difference between itself and the reference voltage of 2.5V becomes maximum, since the A/D-converter uses 0V-5V as the reference voltage, even when the reference value (2.5V) of the knock signal is A/D-converted, the thus converted value does not necessarily become 2.5V which is an intermediate voltage that is determined from the reference voltage of the A/D-converter. In other words, due to the variations or temperature characteristics of circuit elements (resistors, etc.) for generating the reference voltage (2.5V), as illustrated in FIG. 31, an offset occurs between the knock signal reference value and the intermediate voltage of the A/D-converter, with the result that the A/D-converted value of the output knock signal becomes deviated by the extent which corresponds to the offset.
Also, while the output signal from the S/H circuit 8 is A/D-converted in units of, for example, 50 .mu.s and the resulting value is cumulatively calculated by the CPU 3, when the A/D-converters are made common, frequent A/D-conversion processings that are repeatedly executed in units of 50 .mu.s obstruct reliable procurement of time length that is needed for A/D-conversion of other signals.